Selenium is a valuable element of high use in the industrial world since it is known to be useful as a semiconductor material or a photosensitive material and it can also be used as a glass colorant or decolorant.
On the other hand, it is known that selenium becomes toxic if taken in excess, although a trace amount of selenium is an essential element for a human body. Therefore, selenium is an item to be assessed for the environmental standard in order to prevent water pollution and soil pollution.
In such a situation, it may be possible that waste water from factories in the above-mentioned industrial world is contaminated by selenium. Moreover, because a small amount of selenium is also contained in coal, it may also be possible that waste water from thermoelectric power plants is contaminated by selenium. Therefore, the removal of selenium from such waste water has been an important task.
Selenium being a problem in waste water from factories is selenic acid (H2SeO4) which is a hexavalent selenium compound, or a salt thereof, and selenious acid (H2SeO3) which is a tetravalent selenium compound, or a salt thereof. It is considered that most parts of these selenium acid compounds are present in a form of selenate ion (SeO42−) and very small parts thereof are present in a form of selenite ion (SeO32−) in an aqueous solution such as waste water from factories. These selenium acid compounds become zero-valent metallic selenium when reduced.
Incidentally, some microorganisms in the natural world are known to intake these selenium oxyanions into their bodies to carry out a reduction treatment. Specifically, specific kinds of microorganisms, when grown under the presence of selenium oxyanions and a nutrition source, do intake these anions into their bodies and reduce selenate ions into selenite ions as they grow, and furthermore reduce the selenite ions into particulate metallic selenium (for example, refer to Patent Document 1 and Non Patent Document 1). This phenomenon is known as biomineralization, and it is possible with use of this phenomenon to remove selenium oxyanions from a solution while particulating them.
In addition, some extracellular polymeric substances such as polysaccharides, lipids, and vacuoles residing on cell surfaces of microorganisms are able to adsorb selenium oxyanions. This phenomenon is known as bioadsorption. It is said to be possible also with this phenomenon to remove selenium oxyanions from a solution.
The method for removing selenium acid compounds from waste water with the aid of microorganisms in this way can be said to be an excellent method as it imposes less load to the environment.